Hermetically sealed package, and method and machine for manufacturing it

ABSTRACT

A hermetically sealed package comprises an substantially rigid base( 3; 22 ), preferably a tray, a product ( 2; 23 ) to be packaged, which in a preferred embodiment is higher than the tray side walls ( 5 ), supported on the tray ( 3; 22 ), and a stretch film (8; 15) welded to the substantially rigid base ( 3; 22 ) along a sealing line so as to form a hermetically sealed enclosure for the product. 
     In a method and a machine for manufacturing a hermetically sealed package ( 1; 101 ) a stretch film ( 8; 15 ) is stretched by means of a stretching frame ( 17 ) against a base ( 3; 22 ) along a pressure line and is heat welded by means of a sealing frame ( 18 ) to the base ( 3; 22 ) along a sealing line so as to form a hermetically sealed enclosure for a product ( 2; 23 ). (FIG.  4 )

The present invention relates to the packaging of consumer goods (foodand non food products) in plastic containers.

In particular the present invention relates to improved packageconstructions comprising a base over which the product to be packaged isplaced and a thermoplastic cover film which extends over the product andis welded to the base.

A substantial number of products including foodstuffs, such as cheese,meat, processed meat, poultry, fruit, vegetable, fish, pizza, etc., arecurrently sold in packages consisting of a base, such as a flat supportor preferably a tray, on which the product to be packaged is placed,which is then overwrapped with a stretch film, such as, typically,stretch PVC (polyvinyl chloride) and stretch polyolefin films.

This packages are particularly useful when either a flat support is usedas the base or when the base has a tray-like configuration and theproduct to be packaged is higher than the tray side-walls.

A stretch film is by definition a thermoplastic film that when appliedunder tension around a product elongates and conforms to the shape ofthe product to be packaged. Stretch overwrapping is generally carriedout using either a horizontal stretch wrapper or an elevator-typestretch wrapper.

In the horizontal stretch wrapper the film is pre-stretched and appliedover the product while kept under tension by a suitable grip system. Thefilm is then folded longitudinally around the base supporting theproduct and sealed longitudinally below said base by means of acenter-sealer. The film tubing is then transversally severed and thefront and rear flaps thus obtained are folded and welded against thetubing surface by passing the package on a heated belt.

In the elevator-type stretch wrapper, the film is kept tensioned andstretched by raising the product placed on a suitable base against thefilm. Then the film is folded, both transversely and longitudinally,around the base supporting the product and bunch-sealed against thelower surface of said base by passing the package on a heated belt.

Depending on the type of film employed, passing on a heated belt may beinsufficient to close the package by tack welding. In such a case, apressure-assisted welding step is necessary, wherein a driven overheadpressure roller operates in conjunction with the heated belt.

In both cases, however, the welding that is obtained does not alwaysprovide for a hermetically sealed package. As a consequence thereof,purge or in general liquids that exude from the packaged product mayleak from the package and contaminate the outside of the same packageand/or of the other packages that are stored close to it. Furthermorethe presence of a liquid in the tack welding area decreases the strengthof the tack welding and the overwrapping film can easily unwrap oranyway the package becomes loose.

PVC is the film most commonly used in stretch overwrapping because ithas, in addition to a remarkable elongation, also very good elasticproperties, i.e. a good elastic recovery and a very low permanentdeformation.

Alternatively, stretch polyolefin films are commonly employed such asthose described for instance in EP-A-687,558, Japanese patentapplication publication no. 262673/1994 (Derwent Accession Number94-337840), Japanese patent application publication no. 39973/1994(Derwent Accession Number 94-103866), Japanese patent applicationpublication no. 31882/1994 (Derwent Accession Number 94-086225),Japanese patent application publication no. 155210/1985 (DerwentAccession Number 85-239384), or Japanese patent application publicationno. 327936/1992 (Derwent Accession Number 93-002817).

These stretch films may be manufactured by cast extrusion orco-extrusion, using either a flat or a circular film die that allows toshape the polymer melt into a thin film or tube; by heat or gluelamination of two or more cast films obtained as above; or by coating orextrusion coating of a cast film with one or more polymer layers.Alternatively, and preferably, these stretch films are manufactured bythe blown film (or hot blown film) process wherein a mono- ormulti-layer tube is formed and then, while it is still molten, is blownup like a bubble to generate a large diameter tube from a relativelysmall circular die.

Said stretch films, besides having remarkable elongation and preferablygood elastic properties, may also be heat-shrinkable, i.e. they shrinkwhen heated to a temperature that is above the, Vicat softeningtemperature of the film polymers but below their melting temperature.Said heat shrink feature is provided to the stretch films, by theprocess for their manufacture. Said process may involve extruding orco-extruding or extrusion-coating a so-called primary tube or sheet,that is quickly quenched, reheated to a suitable orientation temperatureand oriented either mono-axially or bi-axially (trapped bubble processor tenter frame process). Or, as the so-called double bubble methoddescribed in EP-A-410,792, it may involve extruding or co-extruding themolten polymer to a hot blown film, heating the obtained film to atemperature above its orientation temperature and reinflating it by ablown bubble process. When the stretch film obtained by one of thesemethods is heated to a temperature that approximates the orientationtemperature, it will shrink tending to return to its original dimensionbefore orientation.

Examples of suitable heat-shrinkable stretch polyolefin films are forinstance described in EP-A-286,430, EP-A-369,790, GB-A-2,154,178,EP-A-562,496, U.S. Pat. No. 5,460,861, etc.

Actually, in packaging, the stretch films, either PVC or the stretchpolyolefin films, are used in the same way to overwrap the productplaced on the flat support or in the tray, as indicated above.

When a heat-shrinkable stretch film is employed, a heat treatmentfollowing the bunch-sealing step improves the package appearance bytightly conforming the film to the packaged item. Besides thedisadvantage of the poor hermeticity of the stretch overwrappedconventional packages, the cost of getting a package by stretchoverwrapping, particularly when expensive polyolefin stretch films areemployed, may sometimes be unacceptable at industrial level. A largesurface of film is in fact needed to get

the overwrap of the base and the product placed thereupon, and

the overlapping between the edges to be welded together below the baseitself.

As a consequence of the large surface of film required, an additionaldisadvantage of this packaging method resides in the large amount ofplastic waste that is generated and that eventually needs to be disposedof.

Defensive Publication US-T-896 016 discloses a sealed package comprisinga tray containing a foodstuff and a polymeric film attached to anoutwardly extending flange of said tray by means of a heat sealedadhesive wherein the polymeric film has a tab portion extending beyondat least a portion of the perimeter of the flange to provide easyopening of the package by pulling on the tab.

Said polymeric film has a thickness of about 1 to 10 mils (25,4 to 254μm) and is not stretched over the foodstuff.

As known in the art, a package wherein a film is not stretched over thefoodstuff results in a slack, unattractive package which becomes worseupon handling or upon storage under low temperature conditions.

Further drawbacks of said package are inherent in the manufacturingmethod requiring burdensome and expensive steps

of smearing said heat-sealed adhesive on the tray flange, and

of disposing of a substantial ring portion of film extending beyond theperimeter of the flange to provide material for cutting a tab therein.

FIG. 6 of U.S. Pat. No. 3,587,839 discloses a package consisting of arelatively rigid tray containing a packaged product wherein said tray isclosed between an upper stretched elastic film and a lower heat shrunkfilm. This package involves a large waste of plastic material (i.e. thelower film) compared to a package where the upper film is attached tothe tray rather than to an additional lower film.

In turn, the packaging method disclosed by U.S. Pat. No. 3,587,839comprises the steps of

placing a heat shrinkable film on a platen,

placing a product or a relatively rigid tray containing the product tobe packaged on said heat shrinkable film,

placing a stretchable elastic film on said product or on said relativelyrigid tray containing the product to be packaged,

moving downward a film holding device to hold both films against theplaten along a line around the periphery of said product, or of saidrelatively rigid tray containing the product to be packaged, spacedoutwardly at a predetermined distance therefrom and from the sealinghead so that the tensioning of said upper film occasioned by thesubsequent lowering of said sealing head may be controlled and uniform,

lowering said sealing head while forcing said upper film into contactwith said lower film along a line around and adjacent the periphery ofsaid product or of said relatively rigid tray containing the product tobe packaged,

energizing the electrical resistance of said sealing head to heat bothfilms sufficiently to form a welded seal and to severe them along acontinuous line around and adjacent the periphery of said product or ofsaid relatively rigid tray containing the product to be packaged,

transporting the thus obtained package to a suitable station to effectan appreciable shrinkage of said lower heat shrinkable film.

In said method the sealing head has the double task of tensioning theupper film and of heating both films sufficiently to form a welded sealand to severe them. Thus, it is required sufficient time to allow saidsealing head to become cold before performing the next tensioning a andsealing step otherwise the hot head pierces the upper film whiletensioning it. The packaging speed is thereby substantially slowed down.

U.S. Pat. No. 2,147,384 claims a package comprising a relatively stiffopaque backing sheet of material capable of receiving printingimpressions on the surface threreof, and a transparent facing membraneof thermoplastic elastic material stretched over and attached to theface of said backing sheet along the edge portions of said sheet and amembrane so as to provide a commodity receiving space between said sheetand membrane within the connected edges thereof, the edge portions ofsaid backing sheet being corrugated and the edge portions of saidtransparent membrane being fused into the corrugations of said backingsheet. The backing sheet is said to be preferably composed of acellulose material, preferably coated with casein. In contrast, noinformation at all are given about the composition of said transparentelastic rubber membrane and the elastic modulus thereof.

Even the packaging method and machine applied for manufacturing saidpackage are not enabled by such document.

It is an object of the present invention to provide a hermeticallysealed package for a product supported on a substantially rigid base aswell as a method and a machine for manufacturing said package in a quickway.

It is another object of this invention to provide a hermetically sealedpackage for a product supported on a substantially rigid base as well asa method and a machine for manufacturing said package using less plasticmaterial than necessary according to the prior art.

Preferably, the product to be packaged has a particularly high profileand is placed in a substantially rigid tray.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a hermeticallysealed package comprising:

i) a substantially rigid base,

ii) a product to be packaged supported on said substantially rigid base,

iii) a stretch thermoplastic film extending over the product and welded,all around the product, to said substantially rigid base,

iv) said stretched thermoplastic film being heat welded to saidsubstantially rigid base along a sealing line so as to form ahermetically sealed enclosure for the product.

In a preferred embodiment of the present invention, said substantiallyrigid base is a tray with a hollow or recessed center portion, upwardlyextending side walls and a peripheral rim provided with a continuousflange and said stretched thermoplastic film extends over the productand is heat welded all around the tray along said sealing line on saidcontinuous flange.

In an even more preferred embodiment of the present invention, theproduct to be packaged is higher than the tray side walls.

A second object of the present invention is to provide a method formanufacturing a hermetically sealed package for a product comprising:

a) providing a substantially rigid base,

b) placing said product to be packaged on said substantially rigid base,

c) stretching a thermoplastic stretch film all around said product,

d) welding said thermoplastic stretched film to said substantially rigidbase all around the product,

e) cutting at least some of the stretched film extending beyond theouter perimeter of said substantially rigid base,

f) said stretching being performed by stretching and pressing said filmagainst said substantially rigid base along a pressure line, and

g) said welding of said film to said substantially rigid base beingperformed by heating along a sealing line so as to form a hermeticallysealed enclosure for the product.

In the package and in the method of this invention, preferably thestretch film is stretched in at least one direction by at least 10%,preferably by at least 15%, and even more preferably by at least 20%.Typically, the stretch film is stretched in both transverse andlongitudinal directions. In such a case the stretch film may bestretched to a different degree in one direction compared to the other.

Preferably, when a heat-shrinkable stretch film is employed, said methodfurther comprises heating said package thereby causing theheat-shrinkable stretch film to shrink.

In a preferred embodiment of the method according to the presentinvention the substantially rigid base is a tray with a hollow orrecessed center portion, upwardly extending side walls and a peripheralrim provided with a continuous flange and said stretched thermoplasticfilm extends over the product and is heat welded all around the trayalong said sealing line on said continuous flange.

In a more preferred embodiment of the method according to the presentinvention, the product which is placed on the upper surface of the traycenter portion is higher than the tray side walls.

In an even more preferred embodiment, the product be packaged is freshpoultry and the method of packaging refers to poultry packaging.

A third object of the present invention is to provide a packagingmachine for manufacturing a hermetically sealed package, said packagingmachine comprising:

A) means for feeding a thermoplastic stretch film,

B) means for advancing said film over a substantially rigid base bearinga product to be packaged,;

C) means for stretching said film all around the product;

D) means for welding said stretched film to said substantially rigidbase all around the product; and

E) means for cutting at least some of the stretched film extendingbeyond the outer perimeter of said substantially rigid base, wherein

F) said means for stretching comprises a first frame capable ofstretching and pressing said film against said substantially rigid basealong a pressure line, and

G) said means for welding comprises a second frame capable of heatwelding said stretched film to said substantially rigid base along asealing line so as to form a hermetically sealed enclosure for saidproduct.

As it will be apparent, the present invention overcomes the drawbacks ofthe prior art in that the hermetically sealed package is manufactured byapplying a stretch pressure on the film by means of the first stretchingframe acting along the pressure line and by separately heat welding thefilm by means of a second welding frame acting along the sealing line.Thus, it is avoided any piercing of the film thereby improving thequality of the finished packages and the manufacturing speed is alsoimproved.

Moreover, the waste of plastic material is reduced to a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic side view of a hermetically sealed packageaccording to the present invention, having a product therein. It refersto the preferred embodiment where the base has a tray-like configurationand the packaged product is higher than the tray side walls.

FIG. 2 shows a schematic plan view of the package of FIG. 1 where thebase is a tray of conventional rectangular shape with round comers.

FIG. 3a shows a schematic front view of the equipment for treading-upand of the equipment for stretching the film and lowering it close tothe base on which the product to be packaged is placed, in the packagingmachine of the present invention.

FIG. 3b shows a schematic side view of the equipment of the packagingmachine of FIG. 3a.

The equipment shown in the Figures can suitably be employed for carryingout the first steps in the package forming sequence of the methodaccording to the present invention.

FIG. 3c is a schematic cross-sectional view of the sealing station inthe packaging machine of FIG. 1 where a first equipment for stretchingthe film all around the product and a second equipment for sealing thestretched film to the substantially rigid base are shown. The equipmentshown in this Figure refers to a preferred embodiment where thesubstantially rigid base is a tray and the packaged product is higherthan the tray side walls. It can suitably be employed to complete thepackage forming sequence of the method according to the presentinvention.

FIG. 4 is a schematic enlarged side view, partially in cross-section, ofthe overall packaging machine according to the invention, partiallyshown in FIGS. 3a, 3 b and 3 c.

FIG. 5 is a partial cross-sectional view according to the plane V—V ofFIG. 4.

FIG. 6 is a bottom view of a first stretching frame and of a secondwelding frame of the packaging machine of FIG. 4.

FIGS. 7 and 8 are a side view and a top view, respectively, of thestretching frame of FIG. 6.

DEFINITIONS

As used herein the phrase “over the product” refers to the position of apackage component which is over the product when the product or the traycontaining it is in an upright position.

As used herein the term “liner” refers to a film, laminate, web, orcoating used to line or cover either the upper or lower surface of thebase, corresponding, in case of a tray, to the interior or exteriorsurface thereof. If on the upper interior surface, the liner willtypically be in direct contact with the product. “Interior surface”herein is the surface which forms or defines the space into which theproduct is placed.

As used herein “perimeter” refers to the outer edge, when viewed in planview, of the relevant element, e.g. substantially rigid base, tray,liner or cover stretch film.

As used herein in connection with a multilayer film or sheet, the phrase“outer layer” refers to any layer having less than two of its principalsurfaces directly adhered to another layer of the structure; the phrases“intermediate layer” or “inner layer” refer to any layer having both ofits principal surfaces directly adhered to another layer of thestructure; the term “welding layer” refers to the outer layer that willbe involved in welding of the stretch film to the substantially rigidbase.

As used herein, the phrases “heat-sealable layer” and “heat-sealinglayer” refer to the welding layers of the stretch film and of the baseor base liner (tray or tray liner in the preferred embodiment) that aredirectly adhered one to another, by heating them to at least theirrespective seal initiation temperatures, in the welding step.

The heating can be performed by any one or more of a wide variety ofmanners, such as heated bar(s), hot wires, hot air, infrared radiation,ultrasonic radiation, radio or high frequency radiation, etc., asappropriate.

“Directly adhered”, as used above in connection with the welding layers,is defined herein as the contact between said two layers, one to theother, without an adhesive, glue or any other layer in-between. As usedherein the term “stretch film” refers to a film capable of beingstretched at room temperature (cold stretched) under the conditions ofASTM D-882 (Method A) by at least 150% of its original length withoutbreaking, by applying a stretching force not higher than 2 kg/cm.

As used herein the term “heat-shrinkable” is intended to refer to a filmthat shows at least 5% of free shrink, at least in one direction, whenheated at 90° C. in accordance with ASTM D-2732.

As used herein, the term “polyolefin” refers to any polymerised olefin,which can be linear, branched, cyclic, aliphatic, aromatic, substituted,or unsubstituted. More specifically, included in the term polyolefin arehomopolymers of olefin, copolymers of olefin, copolymers of an olefinand a non-olefinic comonomer copolymerizable with the olefin, such asvinyl monomers, modified polymers thereof, and the like. Included arehomogeneous and heterogeneous polymers. Specific examples includepolyethylene homopolymer, polypropylene homopolymer, polybutene,ethylene-α-olefin copolymer, propylene-α-olefin copolymer,butene-α-olefin copolymer, ethylene-vinyl acetate copolymer,ethylene-ethyl acrylate copolymer, ethylene-butyl acrylate copolymer,ethylene-methyl acrylate copolymer, ethylene-acrylic acid copolymer,ethylene-methacrylic acid copolymer, modified polyolefin resin, ionomerresin, polymethylpentene, etc.

As used herein, the phrase “heterogeneous polymer” refers topolymerisation reaction products of relatively wide variation inmolecular weight and relatively wide variation in compositiondistribution.

As used herein, the phrase “homogeneous polymer” refers topolymerisation reaction products of relatively narrow molecular weightdistribution and relatively narrow composition distribution.

As used herein, the phrase “ethylene-α-olefin copolymer,” is inclusiveof a diverse group of polyethylene copolymers. More specifically, thisphrase encompasses such heterogeneous materials as linear low densitypolyethylene (LLDPE), very low and ultra low density polyethylene (VLDPEand ULDPE), as well as homogeneous polymers such asmetallocene-catalysed EXACT™ linear homogeneous ethylene-α-olefincopolymer resins obtainable from the Exxon Chemical Company, and TAFMER™linear homogeneous ethylene-α-olefin copolymer resins obtainable fromthe Mitsui Petrochemical Corporation. Other ethylene-α-olefincopolymers, such as long chain branched homogeneous ethylene-α-olefincopolymers available from The Dow Chemical Company, known as AFFINITY™resins, are also included as inanother type of homogeneousethylene-α-olefin copolymer.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a package 1 according to the present invention havinga product 2 on a substantially rigid base 3 having a tray configuration.Said tray 3 has a bottom surface 4 whence walls 5 extend, typicallydiverging on the opposite side to the bottom, to a peripheral rim 6 witha flange 7. A stretched film 8 encloses product 2 on tray 3 by weldingto the tray interior surface at flange 7.

FIG. 1 refers to the most preferred embodiment where a product higherthan the tray side walls is packaged.

In FIGS. 3a and 3 b, 10 is a film feed roll, 11 a and 11 b are film rollsupports, 12 is a pinching roll, 13 is an idle roll, and 14 is the chainof grips clamping a film 15.

In FIG. 3c, that also refers to the preferred embodiment where thesubstantially rigid base has a tray-like configuration and the packagedproduct is higher than the tray side walls, 16 is a two-lane machineframe bearing two coupled film stretching frames 17 and two coupledsealing frames 18 with heating elements 19. The stretching frames areraised and lowered by means of pneumatic cylinders 20 and the sealingframes are actuated by pneumatic cylinder 21. The trays are indicatedwith 22, 23 are the products supported on the trays 22, 24 is the traysupport frame, while 15 is the stretched film and 14 are the grips thatadvance the stretched film 15 extending over the products 23 to thesealing station. The tray support frame 24 moves, synchronously with thegrips clamping the stretched film, to advance the corresponding loadedtrays 22 to the sealing station.

FIG. 4 shows the overall packaging machine comprising the equipment ofFIGS. 3a, 3 b and 3 c.

Feeding roll 10 is driven by an electric motor, not shown, at a prefixedspeed for unwinding film 15. While the film is unwound, it is guided bytwo partially arched belts 25 driven by rolls 11 a and 11 b and by theidle roll 13. Film 15 is pulled by grips (not shown in details)supported by two chains 14, as known in the conventional HorizontalStretch Wrapping machines. Said grips are actuated to clamp the filmside edges by pinching roll 12. In turn, chains 14 are driven bypinching roll 12 at a tip speed higher than the tip speed of saidfeeding roll 10 by a preselected amount, thereby stretchinglongitudinally said film 15.

Said two grip chains 14 are guided to and from the sealing station bysprocket couples 26, 27, 28, 29, 30 and 31. The sprockets of each coupleare set at the sides of said two-lane tray support frame 24 (FIGS. 3 cand 5). The sprocket couples 12, 26 and 29 have a distance betweencenters which increases from the sprocket couple 12 to the sprocketcouple 29. Accordingly, the distance between said two grip chains 14increases. In a preferred embodiment, the distance d1 between centers ofsaid sprocket couple 12 is of about 390 mm, while the distance d2between centers of said sprocket couple 29 is of about 400-405 mm. Thus,said film 15 is advanced longitudinally (arrow 32) and stretchedtransversally by said grip chains 14 over the trays 22 and the products23 contained therein.

The tray support frame 24 (FIG. 3c) is carried by a chain 33 driven by asprocket 34.

Said stretching frames 17 and sealing frames 18 of the sealing, stationhave, in plan cross-section, a shape as that of a tray flange 221 (FIGS.5, 6). More particularly, each stretching frame 17 has a peripheral rim171 (FIGS. 3c; 7, 8) which substantially overlaps the inner perimeter(e.g. the inner peripheral rim) of the tray flange 221 and presses saidfilm 15 against said tray flange 221 along a first line (i.e. pressureline). In trun, each sealing frame 18 has a peripheral rim 181 whichpresses said film 15 against said tray flange 221 along a second line(i.e. sealing line) which preferably runs in between said inner andouter flange perimeters.

Thus, said stretching frames 17 bring said film 15 into closed contactwith said flange 221 along a first line and then said hot sealing frame18 enters into closed contact with said film 15 along a separate secondline in order to perform the sealing step by heat welding.

Thereby said film 15 is heat welded to each tray 22 containing theproduct 23, thus obtaining a hermetically sealed packages 101.

The remaining riddled film 15 is released by the grip chains 14 at theoutlet of the sealing station where the grips are actuated by thesprockets 31 to open. The remaining film 15 is then wound on arecovering roll 36, while it is guided by an idle roll 37. In the firststeps of the packaging process that is carried out by the equipmentschematically shown in FIGS. 3a, 3 b and 4, roll 10 is unwound, guidedby the film roll supports 11 a and 11 b and said film 15 is conveyed toroll 12 where it is pinched. Pinching roll 12, runs at a speed higherthan that of roll supports 11 a and 11 b, thus allowing the longitudinalstretching of said film 15. In this step the grips lower the clampedfilm extending it over the product (23) to be packaged loaded on saidsubstantially rigid base 22 as shown in FIG. 3b. Meanwhile, said gripchains 14 cause said film to advance while moving away each other tostretch said film transversely.

When a flat-support is used as substantially rigid base, or when,according to a preferred embodiment of the present invention, asubstantially rigid base having a tray-like configuration, is employedand a product higher than the tray side walls is packaged, said film 15undergoes an additional stretching by extending itself over the productto be packaged 23.

Said substantially rigid base supporting the product to be packaged andsaid stretched film 15 extending over said product, are then advanced tothe sealing station, schematically shown in FIGS. 3c and 4.

The second steps of the packaging process according to the presentinvention can suitably be carried out by means of the equipmentschematically shown in FIGS. 3c and 4. In such an embodiment, stretchingof the cover film all around the products 23 is improved by lowering thestretching frames 17 driven by the cylinders 20 so that their peripheralrims 171 come into close contact with both said film 15 and said innerperipheral rims of the tray flanges 221 along said pressure line. Oncethe stretching frames are in place, the sealing frames 18, heated by theheating elements 19, are lowered by the pneumatic cylinders 21 supportedby the machine frame 16 so that their peripheral rims 181 presse saidfilm 15 on said tray flanges 221 along said sealing lines. The sealingframes are kept in the lowered position at the temperature and for thetime required to heat welding the stretched film to the tray rims.During this time the sealing frames press the stretch film 15 and thetrays 22 against the tray support frame 24.

FIGS. 3c and 4 show a two-lane packaging machine. It has also to beunderstood that the packaging machine of this invention may be designedto comprise a single lane or any other multi-lane without departing fromthis invention. The choice will depend on the number of the products tobe packaged, the machine speed, and the width of the stretch film.

While FIGS. 3c and 4 refer to a preferred, embodiment wherein thestretch film is heat-sealed to the tray flange, it has to be understoodthat the present invention encompasses many suitable sealing frame,capable of joining plastics by ultrasonic waves, by radio or highfrequency welding, etc.

Advantageously, the rigid base is formed of a semi-rigid or, preferably,of a rigid thermoplastic material. These terms, when referred toplastics and plastic sheets, are as defined in ASTM D 883.

Suitable substantially rigid bases can be sheets of foamed or unfoamed,extruded or coextruded or injection moulded materials, or sheets ofcardboard or corrugated cardboard lined with a thermoplastic mono- ormulti-layer film.

When said substantially rigid base has a tray-like configuration, forthe purposes of the present application the term “rigid” identifies atray or container that is self-standing and does not change its shapewhen an item is placed therein, while the term “semi-rigid” identifies atray or container that is self-standing and can be slightly deformed bythe contained item.

A substantially rigid tray can be obtained by thermoforming a foampolymer sheet, such as for instance foam polystyrene (EPS), foampolypropylene (EPP), foam polyester (EPET), and the like foam polymers.These trays are commonly manufactured by a two-stage process thatinvolves extrusion of the polymer foam sheet followed by curing andthermoforming of the polymer foam sheet by methods well known in theart. These foam trays can be pre-formed or formed in line during thepackaging process. If desired, flexible polymeric film sheets can be.adhered thereto to provide for oxygen barrier properties or improvedoxygen barrier properties and or improved sealability. Foam trays with aflexible liner are described e.g. in U.S. Pat. No. 3,748,218, U.S. Pat.No. 3,793,135, U.S. Pat. No. 4,055,672, U.S. Pat. No. 4,076,570, U.S.Pat. No. 4,111,349; U.S. Pat. No. 4,332,858, U.S. Pat. No. 4,558,099,U.S. Pat. No. 4,659,785, U.S. Pat. No. 4,832,775, and U.S. Pat. No.4,847,148. It is also possible, when the liner is used to improvesealability, to adhere it only to the tray flange where the stretch filmhas to be sealed.

These foam polymer trays can also be obtained by injection moulding ofthe foaming polymer.

Alternatively a substantially rigid tray to be used in the manufactureof a package according to the present invention can be made by lining acardboard tray with a mono or multilayer flexible thermoplastic film, asdefined above, that can be welded to the cover stretch film.

Still alternatively a substantially rigid tray can be obtained byinjection moulding of a polymer melt. If barrier properties or improvedbarrier properties are desired, the injection moulded tray can be coatedwith e.g. a PVdC layer, for instance by a spray coating step.

Similarly, a semi-rigid tray may be obtained by thermoforming a mono- ormulti-layer thermoplastic sheet. If oxygen barrier properties aredesired the sheet will comprise at least one layer of an oxygen barriermaterial such as PVdC, EVOH, nylon, EVOH and nylon blends, etc.Depending on the thickness of the starting sheet and the degree of deepdrawing applied, the obtained tray may be more or less thick andtherefore more-or-less substantially rigid. In any case the essentialrequirement for these trays is that the polymer used for the monolayersheet or, in case of a multilayer sheet, the polymer or polymer blendused for the outer layer that will be in contact with the product, canbe welded to the stretch film. Examples of materials for the mono- ormulti-layer sheets that can be thermoformed and used as trays in thepackage according to the present, invention are e.g. polyolefin, PVC,nylon, polyurethane, PVC/polyolefin, polystyrene/polyolefin,nylon/polyolefin polyester/polyolefin, PVC/PVdC/polyolefin,PS/PVdC/polyolefin and many other materials or combinations of materialswhich are well known in the art.

In a preferred embodiment of the present invention, the tray is formedof a foam polymer, preferably foam polystyrene, optionally lined with aheat-sealable polyolefin material, foamed polypropylene or foamedpolyester. In another preferred embodiment the tray is formed of PVC orof a polymer that can be easily welded to a PVC stretch film.

The above description of suitable or preferred materials and methods ofmanufacture that in its wording specifically refers to the preferredembodiment of bases having a tray configuration, does clearly apply alsoto the bases shaped as flat support.

The dimensions of the substantially rigid base are not critical anddepend on the size of the product to be packaged. The substantiallyrigid base should be larger than the product to be packaged leaving anedge of at least 0.5 cm all around the product to allow welding astretch film to a substantially rigid base to get a hermetically sealedpackage.

When a substantially rigid base with a tray-like configuration isemployed, the dimensions of said tray, and particularly of the trayhollow center portion will depend on the size of the product to bepackaged therein. When, according to a preferred embodiment of theinvention, a product higher than the tray side walls is packaged,generally the height of the tray side walls will be from about {fraction(1/20)} to about {fraction (19/20)} of the height of the product to bepackaged.

Also the thickness of the substantially rigid base is not critical forits use in the package and method according to the present invention.

Generally the thickness thereof will depend on the particular type ofthe substantially rigid base (whether a flat support or a tray-likesubstantially rigid base, whether obtained by extrusion or by injectionmoulding, whether made of foamed or unfoamed material, whether mono- ormultilayered, etc.) and the application foreseen. Typically such asubstantially rigid. base will be of from about 150 μm to about 5 mmthick, and preferably of from about 200 μm to about 4 mm thick.

When a substantially rigid base having a tray-like configuration isemployed, particularly when the side walls are inclined and notperpendicular with respect to the tray bottom surface, said tray needsto have a flange all around the tray rim where the stretch film iswelded to the tray. The presence of such a flange will allow in fact theuse of an anvil (the tray support frame indicated as 24 in FIG. 3c) tothe sealing frame that counterpresses the tray against the film to get areliable seal.

The size of this flange is generally sufficient to perform a seal atleast 2 mm wide all around the tray. When the tray side walls areperpendicular to the tray bottom surface, the tray side wall itself willact as an anvil for the sealing frame.

The choice of the material used for the stretch film or for the weldinglayer of the stretch film will be dictated by the choice of the materialused for the upper surface of the substantially rigid base or of thesubstantially rigid base liner and by the method used to weld thestretched film to the substantially rigid base.

In particular, the polymers or polymer blends used for the stretch filmor its welding layer will be chosen so as to heat-seal themselves to thesubstantially rigid base or substantially rigid base liner upper surfaceunder the sealing conditions that are applied.

When according to a preferred embodiment of the present invention an EPSsubstantially rigid base is employed, a stretch film comprising astyrene-based thermoplastic elastomer in the welding layer is preferablyemployed as the cover film and the film and the substantially rigid baseare preferably heat-sealed together by means of a heated framed bypressing them together against a framed anvil.

Styrene-based thermoplastic elastomers that can suitably be employed inthe sealing layer of such a stretch film include but are not limited tostyrene-butadiene block copolymers, styrene-butadiene-styreneterpolymers, styrene-ethylene butene-styrene block terpolymers, andstyrene-isoprene-styrene terpolymers. For use in the sealing layer ofthe stretch film said thermoplastic elastomers may simply be compoundedwith the conventional additives, such as lubricants and slip agents, oralso blended, if desired, with a suitable polymer compatible therewith.

More particularly it has been found that the styrene-butadiene blockcopolymer that is sold by BASF under the trade name Styroflex BX 6104can suitably be employed to manufacture a stretch film that can be heatsealed to an EPS substantially rigid base. Said stretch film may or maynot be also heat-shrinkable.

Also suitable for use in connection with polystyrene bases are the filmscontaining α-olefin/styrene copolymers described in WO 95/32095. Alsosuitable are expected to be the films described in Japanese patentapplication publication 11927/1996 (Derwent Accession Number 96-112039).

Alternatively when the EPS substantially rigid base is lined e.g., witha polyolefin layer such as a polyethylene, an EVA, or a linearpolyethylene layer, a polyolefin stretch film is preferably employed.Stretch polyolefin films that can suitably be used in such a case aree.g. those described in the patent literature listed above. Examples ofsuitable polyolefin stretch films are those sold by Cryovac® under thetrade name SSD 310, a 5-layer, 15 μm thick, symmetrical structure withethylene-vinyl acetate copolymer skin and core layers and low densitylinear polyethylenes intermediate layers, or SES 320, a 5-layer, 15 μmthick, symmetrical structure with a blend of ethylene-vinyl acetatecopolymer and low and medium density linear polyethylenes in the skinlayers, a core layer of ethylene-vinyl acetate copolymer and low densitylinear polyethylenes as intermediate layers.

When a polypropylene substantially rigid base, such as an EPPsubstantially rigid base, is employed, a suitable stretch film that canbe used as the cover film is that commercialised by Asahi under thetrade name H100H. This film which has a high stretchability appears tobe an irradiated 5-layer symmetrical structure with skin layers ofethylene-vinyl acetate copolymer, a core layer of apropylene-butene-ethylene terpolymer and intermediate layers of a blendof ethylene-vinyl acetate copolymer, polypropylene andethylene-propylene copolymer. An alternative commercial film that cansuitably be employed in conjunction with a polypropylene tray isCryovac® SSD 310.

Stretch films such as those described in Japanese patent applicationpublication 304882/1995 (Derwent Accession Number 96-035951) or inJapanese patent application publication 314623/1995 (Derwent AccessionNumber 96-054783) are also expected to be suitable for use inconjunction with a polypropylene substantially rigid base.

When a polyester substantially rigid base, such as an EPET substantiallyrigid base, is employed, suitable stretch films that can be employed asthe cover films have a sealing layer of a polyester or copolyester, morepreferably of a flexible copolyester.

Preferably the thickness of the stretch film will be less than 50 μm.Typically, the stretch films to be used in the package and in the methodof packaging according to the present invention have a thickness of fromabout 8 to about 30 μm and preferably of from about 10 to about 25 μm.

Preferred stretch films to be used in the method and package accordingto the present invention are those than can be cold stretched by atleast 180% of their original length without breaking, by applying astretching force not higher than 2 kg/cm.

More preferred stretch films to be used in the method and packageaccording to the present invention are those than can be cold stretchedby at least 180% of their original length without breaking, by applyingas stretching force not higher than 1.5 kg/cm.

Even more preferred stretch films to be used in the method and packageaccording to the present invention are those than can be cold stretchedby at least 180% of their original length without breaking, by applyinga stretching force not higher than 1 kg/cm.

Furthermore, preferred stretch films are those coupling a highelongation with a low permanent deformation. Permanent deformation of afilm is measured by stretching a sample of the film by 50%, allowing itto relax for 30 seconds and then measuring the percent increase inlength of the sample. A film with a low permanent deformation is a filmthat can recover its original (planar) state after being stretched, suchas by the deforming force of a finger that depresses it. A film with alow permanent deformation will more easily maintain its originalaesthetically attractive appearance even after handling abuse.

Permanent deformation in the two perpendicular directions, TD and MD, ismeasured by an Instron tensile instrument on strips of film 12.5 cm longand 2.5 cm wide. The film is stretched by separating the jaws holdingthe ends of the test specimen at a constant rate until 50% stretch isobtained. Then the jaws are returned to the original position, the filmspecimen is allowed to relax for 30 seconds, and its length is measuredand compared with the original one.

As indicated, preferred stretch films are those than under the aboveconditions show a permanent deformation in both directions lower than20%, and even more preferred are those showing a permanent deformationlower than 15%.

The stretch film may also be heat-shrinkable. In the latter case,typically, the stretch film shows a % free shrink at 90° C. of at least10 in at least one direction. Free shrink is measured:by ASTM MethodD-2732 (5 second immersion time).

For most of the applications the stretch films and the substantiallyrigid bases employed in the package and method according to the presentinvention do not need to have oxygen barrier properties.

However, when oxygen barrier properties are required, oxygen barrierstretch films should be employed in combination with oxygen barrierbases. An example of oxygen barrier stretch film is Ecowrap BSS film byOkura that uses an ethylene-vinyl alcohol copolymer for its oxygenbarrier layer. Suitably plasticised PVdC, e.g. EVA plasticised PVdC,might also be used in the manufacture of oxygen-barrier stretch films.

In the method of packaging according to the present invention, asubstantially rigid base is provided, if necessary or desirable, with athermoplastic film liner adhered by any suitable means to its uppersurface or part thereof. A product, e.g. a fresh poultry product, isplaced onto said substantially rigid base, or in the recess formed bythe tray, when a substantially rigid base with tray-like configurationis employed.

In the preferred embodiment shown in FIGS. 1-4, a substantially rigidbase with tray-like configuration is employed and the packaged productis higher than the tray side walls.

A stretch, optionally heat-shrinkable, film is then stretched over theproduct and welded to the substantially rigid base all around theproduct, or to the tray flange, when a substantially rigid base withtray-like configuration is employed.

In the embodiment shown in FIGS. 3a, 3 b, 3 c, and 4 the film is firststretched longitudinally by running the film roll 10 and the pinch roll12 at a differential speed, then it is stretched transversely e.g. bymeans of a series or chain of gripping means 14 that move apart whilelowering towards the products. When a substantially rigid base isemployed which has the shape of a flat support or has a tray-likeconfiguration and a product higher that the tray side walls is packaged,as shown in FIGS. 3c and 4, an additional stretching is achieved bylowering the gripping means towards the plane of the supported productsas the tensioned film is stretched all around the products.

Once the desired width of the film web is obtained, the gripping meansadvance the film, extended over the products placed on the bases, to thesealing station by moving along parallel tracks. Synchronously also thebases with the products placed thereon are advanced to the sealingstation.

When a substantially rigid base is employed which has the shape of aflat support or has a tray-like configuration and a product higher thatthe tray side walls is packaged, as shown in FIGS. 3c and 4, stretchingof the film all around the product is facilitated by means of astretching frame 17. Said stretching frame is a frame having a perimeterlarger than that of the packaged product and smaller than the outerperimeter of the substantially rigid base, and, in case of asubstantially rigid base with a tray-like configuration, a perimetercontouring the inner perimeter of the tray. When this stretching frameis lowered, it will better conform the stretch film all around theproduct both transversely and longitudinally. While heat welding of thestretched film to the substantially rigid base can be achieved, asindicated before, by any conventional means, in a preferred embodimentsaid heat welding is performed using a heated sealing frame 18. Saidsealing frame 18 has the same shape and a perimeter larger, preferablyonly slightly larger, than that of the stretching frame 17. The sealingframe lowers, once the stretching frame is on place, to seal thestretched film to the substantially rigid base by pressing the filmagainst the substantially rigid base itself.

In the preferred embodiment shown in FIG. 3c, said welding is performedby heat-sealing using a heated sealing frame 18 having the same shapeand the same perimeter as the tray flange. Sealing is achieved bypressing the stretched film against the tray flange supported by thetray support frame 24. In the embodiment shown in FIG. 3c, the sealingframe is heated by means of heating means 19. The sealing temperature issuitably selected depending on the type of material to be sealedtogether as known in the art. Also the pressure exerted on the seal andthe sealing time can be easily set depending on the materials to besealed and the sealing temperature selected.

It is intended that it can be easily conceived to weld the stretch filmto the tray rim by any other conventional heat-sealing means, asindicated above. As an example when materials are employed that can bewelded by RF, instead of the heating elements (19) on the sealing frame,both the sealing frame and the tray support frame (24) may be connectedto a RF generator as known in the art to RF seal the stretch film to thesubstantially rigid base.

The hermetically sealed packages 101 are then separated one from theother by means of a knife, serrated blade, or equivalent means which isbrought down near the outer edge of the substantially rigid base, and,in case of a tray-like substantially rigid base, near the outer edge ofthe tray flange. Some or all of the excess film material extendingbeyond the edge of the substantially rigid base or tray flange is thencut away.

If a heated cutting means and a heat-shrinkable stretch film are used,the heat of the heated knife causes the remaining heat-shrinkablestretch film beyond the seal to shrink-back, close to the seal region,i.e. close to the tray rim or in the flange area-of the tray, when aray-like substantially rigid base is employed, forming a bead along theflange.

When a heat-shrinkable stretch film is employed, the package may beoptionally submitted to a separate heat treatment in order to shrink thestretched film more tightly all around the packaged product.

As indicated above, for most of the applications there is no requirementfor the use of oxygen barrier packaging material and the packagingprocess is carried out under atmospheric pressure. In some cases howeverit may be convenient, in order to improve the shelf life of the packagedproduct, to carry out the packaging process under a suitably modifiedatmosphere obtained e.g. by flushing the modified atmosphere whilestretching lowering the tensioned film against the supported product orby carrying out this step in a closed sealing station.

While FIGS. 3c and 4, as well as the preceding description, generallyrelate to the welding of the stretched film to the upper surface of thetray flange, it would also be possible, alternatively, to stretch thefilm over and around the outer edge of said flange, fold it over thelower surface of said flange and weld it thereto. It would thus bepossible to get a hermetically sealed package and still a saving in theamount of plastic material, even if not as substantial as that obtainedby welding the stretched film to the upper surface of the tray flange asin the preferred embodiment illustrated in FIGS. 3c and 4.

In such a case the packaging machine should also comprise means forfolding the stretched film over the outer edge of the tray flange.Suitable modifications of the packaging machine described above can beeasily devised by the person skilled in the art to carry out theadditional folding step and the welding of the stretched-film-againstthe lower surface of the tray flange.

EXAMPLE 1

Polystyrene foamed trays (23 cm×14.5 cm) with inclined walls about 5 cmhigh and a tray rim with a flange of about 0.8 cm all around the trayperimeter, are loaded with a fresh poultry product extending about 5 cmabove the tray walls. These trays are carried by a conveyor, at apredetermined linear rate, into a two-lane sealing station. The coverfilm is a mono-layer stretch film of Styroflex BX 6104 by BASF, about 15μm thick and 38 cm wide, with elongation of 150% at 0.35 kg/cm stressand a permanent deformation in both directions less than 20%. Said coverfilm is stretched longitudinally by 30% and transversely by 10% by thestretching process described above. An additional percent stretchingwould result from the extending of the film all around the products. Thestretching frames 17, wherein each frame has a perimeter contouring theinner perimeter of the corresponding tray, are lowered to conform thestretch film all around the products both transversely andlongitudinally and once the stretching frames are in place, the heatedsealing frames 18 press the stretch film against the tray flangessupported by the tray support frame. The stretching and the sealingframes are then raised and the sealed packages are advanced. A bladecontouring the tray flanges finally removes the excess of stretch filmbeyond the tray outer edges.

In the sealing step, the sealing pressure is set at 2 kg/cm² and thesealing time at 0.5 sec, while the sealing temperature has been varied.Under these sealing conditions it has been found that reliable seals canbe obtained in the range of temperatures comprised between 80 and 200°C., while the preferred sealing temperature was about 120° C.

EXAMPLE 2

Polystyrene foam trays as in Example 1 are lined with a flexible filmhaving the following structure : a sealant layer (i.e. the layer to besealed to the stretch film cover in the tray flange area) ofethylene-vinyl acetate copolymer (EVA); a tie layer of an anhydride.modified ethylene-vinyl acetate copolymer; and a layer ofethylene-methyl acrylate copolymer for bonding to the foamed polystyrenetray.

The products are placed in the trays and Cryovac® SSD-310 stretch film(a stretch heat-shrinkable 15 μm thick film having the followingstructure: EVA LLDPE EVA LLDPE EVA with 150% elongation at 1.2 kg/cmstress and less than 15% permanent deformation in both directions) isstretched over the products and heat-sealed along the flange at the trayrim. The excess stretch film is cut away by means of a heated cuttingblade and a bead is formed along the flange due to the shrink of theexcess film beyond the seal.

EXAMPLE 3

Trays of the same size as indicated in Example 1 are obtained bythermoforming a PVC sheet 320 μm thick. The trays are loaded with freshpoultry product extending above the tray side walls by about 5 cm, as inExample 1 and a stretch PVC film, 15 μm thick, is stretched over theproducts and heat-sealed to the tray flange by means of the heatedsealing frame under the following conditions: sealing temperature=175°C., sealing pressure=2.0 kg/cm², and sealing time=3 sec.

Alternatively the stretch PVC film may be sealed to the PVC tray flangeby RF under conventional conditions.

In experiments carried out with a product 6-cm higher than the tray sidewalls, stretching was preferably of from about 33 to about 45% intransverse direction and of from about 25 to about 30% in longitudinaldirection; a further increase of the height of the product by 1 cm withrespect to the tray side walls resulted in an additional stretching ofabout 10% in transverse direction and about 5% in longitudinaldirection.

COMPARATIVE EXAMPLE 4

The same trays as in Example 1 are loaded with fresh poultry productextending above the tray side wall by about 5 cm, and overwrapped withthe same stretch film as in Example 1. The edges of the film are thenlongitudinally sealed below-the tray and then the rear, and front flapsare folded over the tray edges and sealed to the tubing surface on thebottom of the tray. The packaging machine was a conventional Omorihorizontal stretch wrapper.

By comparing the amount, in cm², of film required for each package,including the unavoidable scrap, it has been found that by the packagingmethod of the present invention a saving of more than 50% of the stretchfilm is obtained with respect to the conventional process.

What is claimed is:
 1. A hermetically sealed package comprising: a) asubstantially rigid base; b) a product supported on said substantiallyrigid base; and c) a thermoplastic stretch film over said productwherein said stretch film: is capable of cold stretching at roomtemperature under the conditions of ASTM D-882 (Method A) by at least150% of its original length without breaking when applying a stretchingforce of not higher than 2 kg/cm; is capable of a permanent deformationin length in each of the machine and transverse directions of less than20%, where the permanent deformation is measured by providing arepresentative film sample having original dimensions of 12.5 cm longand 2.5 cm wide, stretching the film sample lengthwise at a constantrate until 50% stretch is obtained, releasing the film sample for 30seconds, subsequently measuring the length of the stretched film sample,and calculating the percent increase in length of the stretched filmsample compared to the original film sample; is pre-stretched by anelongation of at least 10% in at least one direction to place thestretch film under pre-stretched tension before contact with said rigidbase or said product; and is welded to said substantially rigid basealong a sealing line while said stretch film is under at least saidpre-stretched tension so as to form said hermetically sealed package. 2.The hermetically sealed package of claim 1 wherein said substantiallyrigid base is a tray with a recessed center portion, upwardly extendingside walls, and a peripheral rim provided with a continuous flange, saidthermoplastic film being heat bonded along said sealing line on saidcontinuous flange.
 3. The hermetically sealed package of claim 2 whereinsaid product extends above said tray side walls.
 4. The hermeticallysealed package of claim 1 wherein said thermoplastic film is capable ofbeing stretched without breaking by at least about 180% of its originallength, as measured according to Method A of ASTM D-882, at roomtemperature when a stretching force of up to about 1 kg/cm is appliedthereto.
 5. The hermetically sealed package of claim 1 wherein saidthermoplastic film is heat shrinkable.
 6. The hermetically sealedpackage of claim 1 wherein said substantially rigid base is a foampolymer, optionally lined with a heat sealable thermoplastic film. 7.The hermetically sealed package of claim 6 wherein said foam polymer isfoamed polystyrene, foamed polypropylene, or foamed polyester.
 8. Thehermetically sealed package of claim 7 wherein said foam polymer isfoamed polystyrene and wherein said thermoplastic film comprises in itsheat-sealing layer a thermoplastic elastomer comprising mer unitsderived from styrene.
 9. The hermetically sealed package of claim 1wherein said product comprises poultry meat.
 10. The package of claim 1wherein said stretch film is pre-stretched by an elongation of at least15% in at least one direction to place the stretch film underpre-stretched tension before contact with said rigid base or saidproduct.
 11. The package of claim 1 wherein said stretch film is capableof cold stretching at room temperature under the conditions of ASTMD-882 (Method A) by at least 180% of its original length withoutbreaking when applying a stretching force of not higher than 1.5 kg/cm.12. A method of manufacturing a hermetically sealed package for aproduct, comprising: a) placing said product on a substantially rigidbase; b) pre-stretching a thermoplastic stretch film by an elongation ofat least 10% in at least one direction to place the stretch film underpre-stretched tension before contact with said rigid base or saidproduct, wherein said stretch film: is capable of cold stretching atroom temperature under the conditions of ASTM D-882 (Method A) by atleast 150% of its original length without breaking when applying astretching force of not higher than 2 kg/cm; and is capable of apermanent deformation in length in each of the machine and transversedirections of less than 20%, where the permanent deformation is measuredby providing a representative film sample having original dimensions of12.5 cm long and 2.5 cm wide, stretching the film sample lengthwise at aconstant rate until 50% stretch is obtained, releasing the film samplefor 30 seconds, subsequently measuring the length of the stretched filmsample, and calculating the percent increase in length of the stretchedfilm sample compared to the original film sample; c) extending saidpre-stretched stretch film over said product and base; d) pressing saidpre-stretched stretch film against said substantially rigid base along apressure line; and e) welding said stretch film to said substantiallyrigid base while said stretch film is under at least said pre-stretchedtension by heating along a sealing line so as to form said hermeticallysealed package.
 13. The method of claim 12 wherein said thermoplasticfilm is heat shrinkable, and further comprising the step of heating saidpackage so as to cause said heat-shrinkable film to shrink.
 14. Themethod of claim 12 wherein said substantially rigid base is a tray witha recessed center portion, upwardly extending side walls, and aperipheral rim provided with a continuous flange, said thermoplasticfilm being heat bonded along said sealing line on said continuousflange, said product optionally extending above said tray side walls.15. The method of claim 12 wherein said product comprises poultry meat.16. The method of claim 12 wherein said pre-stretching step places saidthermoplastic film under pre-stretched tension in both longitudinal andtransversal directions.
 17. The method of claim 12 wherein said weldingstep includes heat sealing said stretch film to said substantially rigidbase.
 18. The method of claim 12 wherein said welding step includesultrasonic wave or radio frequency welding said stretch film to saidsubstantially rigid base.
 19. The method of claim 12 further comprisingthe step of stretching said pre-stretched film beyond said pre-stretchedtension by contacting the film with said product.
 20. A machine formanufacturing a hermetically sealed package, comprising: a) means forfeeding a thermoplastic stretch film; b) means for advancing said filmover a substantially rigid base bearing a product to be packaged whilepre-stretching said film by an elongation of at least 10% in at leastone direction to place the film under pre-stretched tension beforecontact with said rigid base or said product; c) a first frame capableof pressing said film against said substantially rigid base along apressure line; and d) a second frame capable of welding saidthermoplastic film to said substantially rigid base along a sealing linewhile said film is under at least said pre-stretched tension so as toform a hermetically sealed package.
 21. The machine of claim 20 whereinsaid advancing means comprises a pinching roll and two partiallydiverging grip chains.